Patients requiring artificial ventilation are usually connected to lung ventilators administering in cycles, by means of positive pressure, tidal volumes of a mixture composed by air and oxygen. In an acute respiratory failure condition, the patient is usually sedated or in apnea state, i.e. there is no breathing effort and mechanical ventilation is made by means of controlled cycles.
Controlled cycles may usually be volume controlled or pressure controlled. Volume controlled cycles present a fixed flow pattern and any change in the respiratory mechanics results in change of the pressure in the patient's airway. Controlled pressure mode, on the other hand, keeps pressure in the patient's airway constant, presenting free demand flow and the volume is consequently variable.
When the patient shows any breathing effort, cycles may be synchronized with the patient's efforts and are then designated as assisted cycles. In case of assisted cycles, the controlled volume mode, despite being synchronized with the patient's initial effort, does not allow flow synchronization along the cycle, thus resulting in discomfort for the patient. On the other hand, the controlled pressure mode allows the patient to modulate the inspiratory flow, but does not ensure the resulting tidal volume is appropriate for proper respiratory support.
In one stage when the patient has better control over ventilation, spontaneous cycles may be made available through the ventilator. Usually, these cycles are partially assisted by the ventilator by means of a pressure support similar to the one used in the controlled pressure mode. Also in this case, the lack of control over the volume is a critical point, especially in patients at the initial phase of being weaned off ventilator support.
Clinical studies show the existence of a natural variability pattern in spontaneous or assisted breathing by patients, including newly born patients, and its potential beneficial effects. The variability of the respiratory pattern is linked e.g. to the success ventilator prosthesis weaning, besides a positive repercussion in respiratory mechanics and gas exchanges.
This variability pattern is practically suppressed by the use of the state of art ventilation modes, wherein, by means of various techniques, a pre-defined target volume is attempted to be reached and maintained.
Considering the above explanations, it is clear that there is a need to improve current ventilation modes, so to, at the same time, assure that safe ventilation limits are maintained and preserve the natural variability as present in patient's spontaneous breathing.